[REVIEW ONLY] Landing livedev2 in Brackets 

This is just an initial commit for moving livedev2 extension to Brackets core as per in njx/brackets-livedev2#24. Current implementation was moved to ```LiveDevelopment/impls/default```. ```livedev2``` implementation was added to ```LiveDevelopment/impls/livedev2```

They can be switched by a preference named: ```livedev.impl``` which is being checked at ```LiveDevelopment/main.js```.

@busykai, please review this initial commit to validate overall approach and ```PreferencesManager``` usage. It seems that is not retrieving pref value when manually setting it at project level in ```.brackets.json```